In a variety of clinical situations it is important to measure certain chemical characteristics of the patient's blood such as pH, hematocrit, the concentration of calcium ions, potassium ions, chloride ions, sodium ions, glucose, lactate, creatinine, urea, the partial pressure of oxygen (O2), carbon dioxide (CO2), and the like. These situations range from a routine visit of a patient in a physician's office to monitoring of a patient during open-heart surgery. The required speed, accuracy, and other performance characteristics vary with each situation.
Typically, electrochemical sensor systems which provide blood chemistry analysis are stand-alone machines or are adapted to be connected to an extracorporeal shunt or an ex vivo blood source such as a heart/lung machine used to sustain a patient during surgery. Thus, for example, small test samples of ex vivo blood can be diverted off-line from either the venous or arterial flow lines of a heart/lung machine directly to a chamber exposed to a bank of micro-electrodes which generate electrical signals proportional to chemical characteristics of the real time flowing blood sample.
Electrochemical sensor systems are analytical tools combining a chemical or biochemical recognition component (e.g., an enzyme) with a physical transducer such as a platinum electrode. The chemical or biochemical recognition component is capable of selectively interacting with an analyte of interest and of generating, directly or indirectly, an electrical signal through the transducer. Electrochemical sensor systems play an increasing role in solving analytical and clinical problems, and find applications in the field of medical diagnostics.
The selectivity of certain biochemical recognition components makes it possible to develop electrochemical sensors that can accurately detect certain biological analytes even in a complex analyte mixture such as whole blood. Despite the high degree of selectivity of electrochemical sensors, the accuracy of such sensors depends on calibrating the sensor against a calibration solution that includes a known concentration of analyte. If the actual concentration of the analyte in the calibration solution is different from the concentration assumed to be in the solution, then the sensor may be improperly calibrated and the sensor readings may be inaccurate. The oxygen in a calibration solution may be removed by, for example, the oxidation of other components of the solution or contaminants in the solution. The amount of oxidation that occurs in the calibration solution is unpredictable and, over a period of time, may lead to an oxygen content in the calibration solution different from the oxygen content estimated to exist from the preparation of the solution. A calibration solution in which the oxygen content is unknown and may be different from the estimated value may lead to an incorrect calibration of the sensor and inaccurate sensor readings.